Sound-reproducing audio systems are used to pick up sound at one location and reproduce it at the same or some other location. Audio systems may be generally classified as monaural, binaural, stereophonic, or quadraphonic. The most commonly used systems are monaural and stereophonic.
Monaural ("mono") audio systems transmit sound signals on a single transducing channel connected to one or more speakers and/or earphones. The most common example is the telephone, which generally has one microphone, one transducer and one earphone.
Stereophonic ("stereo") audio systems are field-type systems in which two or more microphones are used to pick up the original sound. Each microphone is coupled to one of at least two independent transducing channels (L and R) which are themselves coupled to at least two loudspeakers. In the ideal configuration, the loudspeakers are arranged in the same general position as the microphones. The stereo transducer may be an amplifier, a radio transmitter, a radio receiver, a phonograph, a sound motion picture unit, a television transmitter and/or receiver, or a magnetic tape recorder and reproducer.
Repercussion audio systems (also known as "Surround-Sound") have been developed by the assignee of the present invention for creating realistic sounds in a conventional stereo system. For example, in a live setting such as a concert hall or a theater, listeners receive direct sounds and repercussion sounds. The direct sounds emanate from the original sound source and/or the loudspeakers, and the repercussion sounds are direct sounds that reflect off walls or other objects in the hall to the listener. Repercussion audio systems provide, in addition to the left and right stereo speakers, an extra set of repercussion speakers for producing repercussion sounds. The typical repercussion system utilizes phase delays and other features to simulate the repercussion sounds a listener would hear in a concert hall or theater.
FIG. 1 illustrates a frequency-modulated stereo transmission signal. The modulated stereo signal generally includes a L+R component, a pilot carrier, a L-R component and a suppressed carrier. It is not practical to simply modulate L and R separately because the transmitted signal must be decipherable by either a stereo or mono receiver. If L and R were modulated independently, the mono receiver could receive only one channel or the other. For the mono receiver to correctly receive a stereo broadcast, some component of that broadcast must be a single signal that includes information from both the left and the right channels. This single signal is the L+R signal shown in FIG. 1. The mono receiver simply demodulates L+R and delivers it to the listener.
A stereo receiver uses the L+R signal and the L-R signal to demodulate separate L and R signals. The stereo receiver separates L and R according to the following equations: EQU (L+R)+(L-R)=2L *1* EQU (L+R)-(L-R)=2R *2*
As shown in FIG. 1, stereo broadcasts also includes a pilot carrier signal (15.75 kHz) which indicate to the receiver that the broadcast is in stereo. However, in applications such as radio and television broadcasts, the pilot carrier may accompany both mono and stereo signals. For example, commercials are typically broadcast in mono even though the actual program is in stereo. However, broadcasters typically do not bother to turn off the pilot carrier signal during commercials, and thus, the stereo receiver produces identical signals (L+R) at each stereo speaker.
FIG. 2 illustrates an example of a stereo system 20 used in connection with a television receiver (not shown). For the example shown in FIG. 2, the stereo system 20 includes an FM demodulator 22, a repercussion circuit 24, a power amplifier 26, a speaker system 28, and a microprocessor 30. The FM demodulator 22 receives signals from an intermediate-frequency amplifier (not shown) and outputs left (L) and right (R) audio signals to the repercussion circuit 24. A pilot carrier signal (shown in FIG. 1) is transmitted with stereo broadcasts in order to notify the FM demodulator 22 that the incoming signals are stereo. If the pilot carrier is not present, the FM demodulator 22 assumes that the incoming signals are mono. The FM demodulator 22 informs the microprocessor 30 of whether the incoming signal is mono or stereo, and the microprocessor 30 chooses the appropriate mode of operation (stereo or mono). The microprocessor 30 also allows the user to chose various configurations for the television's audio system such as stereo, mono or repercussion modes. An example of a television with a repercussion mode and microprocessor control is the model CTP-3180 by Matsushita Electronics Corp. of America.
FIG. 3 is a more detailed diagram of the repercussion circuit 24 shown in FIG. 2. Amplifiers 32 and 34 amplify the incoming L and R signals from the FM demodulator 22 (shown in FIG. 2), and a difference amplifier 36 produces an output signal (L-R) proportional to the difference between the incoming L and R signals. The output from the difference amplifier 36 is fed to a phase shift network 38 in order to generate a phase shifted signal .phi.(L-R). The signal .phi.(L-R) is added to L by a summing amplifier 26 to produce L+.phi.(L-R). The signal .phi.(L-R) is inverted by a phase invertor 42 and also added to R by a summing amplifier 44 to produce R-.phi.(L-R). The power amplifier 26 amplifies the outputs from the summing amplifiers 40, 44 to produce A(L+.phi.(L-R)) for the left channel 46 and A(R-.phi.(L-R) for the right channel 48. The .phi.(L-R) component represents the simulated repercussion sounds heard in concert halls. The speaker system 28 includes a left stereo speaker 50, a right stereo speaker 52 and repercussion speakers 54. The left channel 46 is connected to the positive (+) input of the left speaker 50, and the right channel 48 is connected to the positive (+) input of the right speaker 52. The Repercussion speakers 54 are connected in series across the positive inputs of the stereo speakers 50, 52.
The repercussion speakers 54 are driven by the potential difference from the positive input of the left stereo speaker 50 to the positive input of the right stereo speaker 52. This potential is calculated as follows: EQU A(L+.phi.(L-R))-A(R-.phi.(L-R)) =A(L-R)+2A .phi.(L-R) *3*
The repercussion speakers 54 are connected in series and have substantially the same resistance, and thus the total potential drop across both speakers 54 is divided equally across each of the speakers 54. Accordingly, the voltage potential Vp for driving each repercussion speaker 54 is as follows: EQU Vp=1/2A(L-R)+A.phi.(L-R) *4*
As shown by equation *4* above, the voltage drop across the repercussion speakers 54 should be zero when L and R are identical. However, L and R always have some amplitude and phase differences due to uneven transmission characteristics in the left and right channels 46, 48. Thus, when a mono signal is received by a repercussion stereo, noises may be generated in correspondence with the random amplitude and phase differences between the left and right channels.
Thus, there is a need for a stereo system in which stereo signals are detected directly regardless of whether a pilot carrier signal is present. There is a further need for a repercussion stereo system that avoids the noises that may be generated when a repercussion stereo receives a mono signal. To date, no circuit or method has been provided for addressing these and other associated needs.